This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-335450, filed Oct. 31, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an injection unit of an electric injection molding machine, which is used to plasticize raw material resin and fill dies with the plasticized resin and, particularly, to the construction of a drive mechanism to advance a screw in a heating barrel.
2. Description of the Related Art
The construction of an injection unit in a conventional electric injection molding machine is described, for example, in Jpn. Pat. Appln. KOKAI Publication No. 10-286848 or Published Japanese Patent No. 2631521.
FIG. 2 shows, as an example of the construction of a conventional injection unit, the construction described in the Jpn. Pat. Appln. KOKAI Publication No. 10-286848. This injection unit comprises a heating barrel 1, a screw 2, a front plate 3, a rear plate 4, an intermediate plate 6, etc.
The heating barrel 1 is, on the leading end thereof (the left side in the drawing), connected to the rear surface of dies (not shown) through a nozzle (not shown). The screw 2 is incorporated in the heating barrel 1. The tail end of the heating barrel 1 is supported by the middle of the front plate 3. Behind the front plate 3 is arranged the rear plate 4 opposite to the front plate 3. Four tie bars 5 span the space between the front plate 3 and the rear plate 4, which are connected to each other through these tie bars 5.
Between the front plate 3 and the rear plate 4 is arranged the intermediate plate 6. Through holes are provided at four corners of the peripheral edge portion of the intermediate plate 6, and the tie bars 5 pass through these through holes. The intermediate plate 6 can move in a back-and-forth direction (a left-and-right direction in the figure) while being guided by the tie bars 5.
A bearing 7 is built in the middle of the intermediate plate 6 and the tail end of the screw 2 is supported by this bearing 7. Furthermore, a servomotor 9 for charging is placed on the intermediate plate 6. A pulley 8 is attached to the tail end portion of the screw 2. This pulley 8 is connected to the rotating shaft of the servomotor 9 through a timing belt.
Nuts 32 of ball screws are fixed in two places of the peripheral edge portion of the rear surface of the intermediate plate 6. Bearings 34 are built in two places of the peripheral edge portion of the rear plate 4, and the threaded rods 31 of the ball screws are supported by these bearings 34. The tail end portion of the threaded rod 31 of each ball screw protrudes from the rear surface of the rear plate 4 and a pulley 36 is attached to this portion. These pulleys 36 are connected to a servomotor for injection (not shown) through a timing belt. Moreover, in this example, a pressure sensor 35 to detect a reaction force that acts on the screw 2 during injection is attached to the middle of the rear surface of the intermediate plate 6.
In the injection unit, by rotating the screw 2 by use of the servomotor 9, a raw material resin is introduced into the heating barrel 1 and melted. Also, the molten resin is injected from the heating barrel 1 into the dies by driving the intermediate plate 6 by means of the ball screws 30 thereby to advance the screw 2 in the heating barrel 1.
In a case where, as in the example, the nut 32 of the ball screw is attached to the rear surface of the intermediate plate 6, the length of the nut 32 has a direct effect on the total length of the injection unit. That is, when the length of the nut 32 is increased, the total length of the injection unit also increases by this amount.
Incidentally, in an injection unit of an electric injection molding machine, high response is required during the control of the advance speed of the screw 2. Therefore, it is necessary to minimize the inertia of an axial driving mechanism of the screw 2 (generally, ball screws 30 are used as in the example). For this purpose, it is desirable to use ball screws of the smallest possible diameter.
On the other hand, in order to withstand the thrust during injection, it is necessary to increase the number of balls that transmit force between the threaded rod and the nut. In order to increase the number of balls, it is necessary to increase the number of turns and the number of rows. However, if this is carried out, the length of the nut increases. In order to shorten the length of the nut, it is necessary to increase the number of balls per perimeter by increasing the diameter of the threaded rod. However, if this is carried out, the inertia increases and the response during the acceleration and deceleration of the screw becomes worse.
Furthermore, in a case where, as in the example, means for detecting the back pressure of the screw 2 (the pressure sensor 35) is provided in addition to the ball screws 30 for the axial driving of the screw 2, it is impossible to individually detect the load of each of the ball screws 30. For this reason, when an excessive force acts on one ball screw, it is impossible to detect the abnormality and the ball screw may be broken early. Incidentally, in the described Published Japanese Patent No. 2631521, this problem is solved by inserting back-pressure detection means in the connection between the nut of each ball screw and the intermediate plate.
When the axial driving mechanism of the screw 2 is constituted by a plurality of ball screws 30, the axis of the screw 2 and the axis of each of the ball screws 30 are not on the same straight line, as shown in FIG. 3, and therefore bending moment acts on the intermediate plate 6, thereby generating deflection. Because this bending moment acts on the ball screws 30 as a lateral load, an excessive force acts on the balls kept between the nut 32 and the threaded rod 31, shortening the life of the ball screws 30.
To solve this problem, it is conceivable to decrease the amount of deformation of the intermediate plate 6 by increasing the rigidity of the intermediate plate. However, if the thickness of the intermediate plate 6 is increased, the total length of the injection unit also increases by this amount.
The present invention was made in view of the problems with the injection unit of a conventional injection molding machine. The object of the invention is to provide an injection unit which enables the total length of the unit to be shortened, is excellent in response during the control of the advance speed of a screw, and has a long life of an axial driving mechanism of the screw.
The injection unit of an electric injection molding machine of the invention comprises: a heating barrel having a nozzle at the leading end thereof; a screw, the leading end side of which being incorporated in the heating barrel; a front plate supporting the tail end of the heating barrel; a rear plate arranged behind the front plate opposite to the front plate; tie bars spanning a space between the front plate and the rear plate and connecting both together; an intermediate plate arranged between the front plate and the rear plate, moving along the tie bars in a back-and-forth direction and rotatably supporting a vicinity of the tail end of the screw; a rotationally driving mechanism mounted to the intermediate plate and configured to rotate the screw; and ball screws mounted to the rear plate and configured to drive the intermediate plate in a back-and-forth direction. In this injection unit of an electric injection molding machine, the nut of each of said ball screws is rotatably attached to said rear plate, through holes are formed in said intermediate plate, the leading end portion of the threaded rod of each of said ball screws and the leading end portion of the nut are inserted in each of these through holes, and the leading end of the threaded rod is connected to the front surface side of said intermediate plate.
According to the injection unit of an electric injection molding machine of the invention, because the leading end portion of the nut of each of the ball screws is inserted in each of the through holes, it is unnecessary to increase the total length of the injection unit even in a case where the length of the nut is increased in order to increase the injection pressure. Furthermore, the thickness of the intermediate plate can be increased without increasing the total length of the injection unit. Therefore, it is possible to adopt the measure to increase the stiffness of the intermediate plate by increasing the thickness of the intermediate plate thereby to reduce the deflection of the intermediate plate. As a result, the bending moment acting on the ball screws can be minimized and the life of the ball screws can be extended.
Preferably, load cells are fixed to the front surface of the intermediate plate so as to block the front of the through holes and the leading end of each of the threaded rods is fixed to the rear surface of each of the load cells.
In this manner, by inserting a load cell in the connection between each of the threaded rods and the intermediate plate, it becomes possible to directly detect a load applied to each of the threaded rods. Furthermore, because the load cells are arranged on the front surface side of the intermediate plate, the total length of the injection unit does not increase due to the mounting of the load cells.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.